Method and apparatus using cell-specific and common pilot subcarriers in multi-carrier, multi-cell wireless communication networks

ABSTRACT

A multi-carrier cellular wireless network ( 400 ) employs base stations ( 404 ) that transmit two different groups of pilot subcarriers: (1) cell-specific pilot subcarriers, which are used by a receiver to extract information unique to each individual cell ( 402 ), and (2) common pilots subcarriers, which are designed to possess a set of characteristics common to all the base stations ( 404 ) of the system. The design criteria and transmission formats of the cell-specific and common pilot subcarriers are specified to enable a receiver to perform different system functions. The methods and processes can be extended to other systems, such as those with multiple antennas in an individual sector and those where some subcarriers bear common network/system information.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of, and incorporates herein byreference in its entirety, U.S. patent application Ser. No. 13/874,278,entitled “METHODS AND APPARATUS USING CELL-SPECIFIC AND COMMON PILOTSUBCARRIERS IN MULTI-CARRIER, MULTI-CELL WIRELESS COMMUNICATIONNETWORKS,” filed Apr. 30, 2013, which is a continuation of, andincorporates herein by reference in its entirety, U.S. patentapplication Ser. No. 13/212,116, now granted U.S. Pat. No. 8,432,891,entitled “METHODS AND APPARATUS USING CELL-SPECIFIC AND COMMON PILOTSUBCARRIERS IN MULTI-CARRIER, MULTI-CELL WIRELESS COMMUNICATIONNETWORKS,” filed Aug. 17, 2011, which is a continuation of, andincorporates herein by reference in its entirety, U.S. patentapplication Ser. No. 10/583,530, now granted U.S. Pat. No. 8,009,660,entitled “METHODS AND APPARATUS USING CELL-SPECIFIC AND COMMON PILOTSUBCARRIERS IN MULTI-CARRIER, MULTI-CELL WIRELESS COMMUNICATIONNETWORKS,” filed May 30, 2007 which is a U.S. National Stage of PCTApplication No. PCT/US05/01939, entitled “METHODS AND APPARATUS FORMULTI-CARRIER, MULTI-CELL WIRELESS COMMUNICATION NETWORKS,” filed Jan.20, 2005, which claims the benefit of and priority to U.S. ProvisionalPatent Application No. 60/540,032, entitled “METHODS AND APPARATUS FORMULTI-CARRIER, MULTI-CELL WIRELESS COMMUNICATION NETWORKS,” filed onJan. 29, 2004.

BACKGROUND

In multi-carrier wireless communications, many important systemfunctions such as frequency synchronization and channel estimation,depicted in FIG. 1, are facilitated by using the network informationprovided by a portion of total subcarriers such as pilot subcarriers.The fidelity level of the received subcarriers dictates how well thesefunctions can be achieved, which in turn affect the efficiency andcapacity of the entire network.

In a wireless network, there are a number of base stations, each ofwhich provides coverage to designated areas, normally called a cell. Ifa cell is divided into sectors, from a system engineering point of vieweach sector can be considered a cell. In this context, the terms “cell”and “sector” are interchangeable. The network information can becategorized into two types: the cell-specific information that is uniqueto a particular cell, and the common information that is common to theentire network or to a portion of the entire networks such as a group ofcells.

In a multi-cell environment, for example, the base station transmitterof each cell transmits its own pilot subcarriers, in addition to datacarriers, to be used by the receivers within the cell. In such anenvironment, carrying out the pilot-dependent functions becomes achallenging task in that, in addition to the degradation due tomultipath propagation channels, signals originated from the basestations at different cells interfere with each other.

One approach to deal with the interference problem has been to have eachcell transmit a particular pattern of pilot subcarriers based on acertain type of cell-dependent random process. This approach, to acertain degree, has mitigated the impact of the mutual interferencebetween the pilot subcarriers from adjacent cells; however, it has notprovided for a careful and systematic consideration of the uniquerequirements of the pilot subcarriers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a basic multi-carrier wireless communication systemconsisting of a transmitter and a receiver.

FIG. 2 shows basic structure of a multi-carrier signal in the frequencydomain, which is made up of subcarriers.

FIG. 3 shows a radio resource divided into small units in both thefrequency and time domains: subchannels and time slots.

FIG. 4 depicts a cellular wireless network comprised of multiple cells,in each of which coverage is provided by a base station (BS).

FIG. 5 shows pilot subcarriers divided into two groups: cell-specificpilot subcarriers and common pilot subcarriers.

FIG. 6 is an embodiment of pilot-generation-and-insertion functionalblock shown in FIG. 1, which employs a microprocessor to generate pilotsubcarriers and insert them into a frequency sequence contained in theelectronic memory.

FIG. 7 shows that common pilot subcarriers are generated by amicroprocessor of FIG. 6 to realize phase diversity.

FIG. 8 is an embodiment of delay diversity, which effectively createsphase diversity by adding a random delay time duration, either inbaseband or RF, to the time-domain signals.

FIG. 9 shows two examples for extension to multiple antennaapplications.

FIG. 10 is an embodiment of synchronization in frequency and timedomains of two collocated base stations sharing a common frequencyoscillator.

FIG. 11 is an embodiment of synchronization in frequency and timedomains with base stations at different locations sharing a commonfrequency reference signal generated from the GPS signals.

FIG. 12 is an embodiment depicting a wireless network consisting ofthree groups of cells (or sectors) and base stations in each group thatshare their own set of common pilot subcarriers.

FIG. 13 shows all base stations within a network transmit, along with acommon pilot subcarrier, a data subcarrier carrying data informationcommon to all cells in the network.

DETAILED DESCRIPTION

In the following description the invention is explained with respect tosome of its various embodiments, providing specific details for athorough understanding and enablement. However, one skilled in the artwill understand that the invention may be practiced without suchdetails. In other instances, well-known structures and functions havenot been shown or described in detail to avoid obscuring the depictionof the embodiments.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Whenthe claims use the word “or” in reference to a list of two or moreitems, that word covers all of the following interpretations of theword: any of the items in the list, all of the items in the list and anycombination of the items in the list.

FIG. 1 depicts a basic multi-carrier wireless communication systemconsisting of a transmitter 102 and a receiver 104. A functional block106 at the transmitter, called Pilot generation and insertion, generatespilot subcarriers and inserts them into predetermined frequencylocations. These pilot subcarriers are used by the receiver to carry outcertain functions. In aspects of this invention, pilot subcarriers aredivided into two different groups according to their functionalities,and hence their distinct requirements. The transmission format of eachgroup of pilot subcarriers will be designed so that it optimizes theperformance of the system functions such as frequency synchronizationand channel estimation.

The first group is called “cell-specific pilot subcarriers,” and will beused by the receiver 104 to extract information unique to eachindividual cell. For example, these cell-specific pilot subcarriers canbe used in channel estimation where it is necessary for a particularreceiver to be able to differentiate the pilot subcarriers that areintended for its use from those of other cells. For these pilotsubcarriers, counter-interference methods are necessary.

The second group is termed “common pilot sub-carriers,” and are designedto possess a set of characteristics common to all base stations of thesystem. Thus, every receiver 104 within the system is able to exploitthese common pilot subcarriers to perform necessary functions withoutinterference problem. For instance, these common pilot subcarriers canbe used in the frequency synchronization process, where it is notnecessary to discriminate pilot subcarriers of different cells, but itis desirable for the receiver to combine coherently the energy of commonpilot subcarriers with the same carrier index from different cells, soas to achieve relatively accurate frequency estimation.

Aspects of this invention provide methods to define the transmissionformats of the cell-specific and common pilot subcarriers that enable areceiver to perform different system functions. In particular, a set ofdesign criteria are provided for pilot subcarriers. Other features ofthis invention further provide apparatus or means to implement the abovedesign processes and methods. In particular, signal reception can beimproved by manipulating phase values of the pilot subcarriers and byusing power control.

The methods and processes can also be extended to other cases, such aswhere multiple antennas are used within an individual sector and wheresome subcarriers are used to carry common network/system information.Base stations can be synchronized in frequency and time by sharing acommon frequency oscillator or a common frequency reference signal, suchas the one generated from the signals provided by the Global PositioningSystem (GPS).

Multi-Carrier Communication System

In a multi-carrier communication system such as multi-carrier codedivision multiple access (MC-CDMA) and orthogonal frequency divisionmultiple access (OFDMA), information data are multiplexed on subcarriersthat are mutually orthogonal in the frequency domain. In effect, afrequency selective channel is broken into a number of parallel butsmall segments in frequency that can be treated as flat fading channelsand hence can be easily dealt with using simple one-tap equalizers. Themodulation/demodulation can be performed using the fast Fouriertransform (FFT).

In a multi-carrier communication system the physical media resource(e.g., radio or cable) can be divided in both the frequency and the timedomains. This canonical division provides a high flexibility and finegranularity for resource sharing. The basic structure of a multi-carriersignal in the frequency domain is made up of subcarriers, and within aparticular spectral band or channel there are a fixed number ofsubcarriers. There are three types of subcarriers:

1. Data subcarriers, which carry information data;

2. Pilot subcarriers, whose phases and amplitudes are predetermined andmade known to all receivers and which are used for assisting systemfunctions such as estimation of system parameters; and

3. Silent subcarriers, which have no energy and are used for guard bandsand DC carriers.

The data subcarriers can be arranged into groups called subchannels tosupport multiple access and scalability. The subcarriers forming onesubchannel are not necessarily adjacent to each other. This concept isillustrated in FIG. 2, showing a basic structure of a multi-carriersignal 200 in the frequency domain, which is made up of subcarriers.Data subcarriers can be grouped into subchannels in a particular way.The pilot subcarriers are also distributed over the entire channel in aparticular way.

The basic structure of a multi-carrier signal in the time domain is madeup of time slots to support multiple-access. The resource division inboth the frequency and time domains is depicted in FIG. 3 which shows aradio resource divided into small units in both the frequency and timedomains: subchannels and time slots, 300. The basic structure of amulti-carrier signal in the time domain is made up of time slots.

As depicted in FIG. 1, in a multi-carrier communication system, ageneric transmitter may consist of the following functional blocks:

1. Encoding and modulation 108

2. Pilot generation and insertion 106

3. Inverse fast Fourier transform (IFFT) 110

4. Transmission 112

And a generic receiver may consist of the following functional blocks:

1. Reception 114

2. Frame synchronization 116

3. Frequency and timing compensation 118

4. Fast Fourier transform (FFT) 120

5. Frequency, timing, and channel estimation 122

6. Channel compensation 124

7. Decoding 126

Cellular Wireless Networks

In a cellular wireless network, the geographical region to be servicedby the network is normally divided into smaller areas called cells. Ineach cell the coverage is provided by a base station. Thus, this type ofstructure is normally referred to as the cellular structure depicted inFIG. 4, which illustrates a cellular wireless network 400 comprised ofmultiple cells 402, in each of which coverage is provided by a basestation (BS) 404. Mobile stations are distributed within each coveragearea.

A base station 404 is connected to the backbone of the network via adedicated link and also provides radio links to mobile stations withinits coverage. A base station 404 also serves as a focal point todistribute information to and collect information from its mobilestations by radio signals. The mobile stations within each coverage areaare used as the interface between the users and the network.

In an M-cell wireless network arrangement, with one-way or two-waycommunication and time division or frequency division duplexing, thetransmitters at all the cells are synchronized via a particular meansand are transmitting simultaneously. In a specific cell 402 of thisarrangement, the pth cell, a receiver receives a signal at a specificsubcarrier, the ith subcarrier, at the time t_(k), which can bedescribed as:

$\begin{matrix}{{s_{i}\left( t_{k} \right)} = {{{a_{i,p}\left( t_{k} \right)}^{{j\phi}_{i,p}{(t_{k})}}} + {\sum\limits_{\underset{m \neq p}{m = 1}}^{M}\; {{a_{i,m}\left( t_{k} \right)}^{{j\phi}_{i,p}{(t_{k})}}}}}} & (1)\end{matrix}$

where a_(i,m) (t_(k)) and φ_(i,m)(t_(k)) denote the signal amplitude andphase, respectively, associated with the i^(th) subcarrier from the basestation of the m_(th) cell.

Cell-Specific Pilot Subcarriers

If the ith subcarrier is used as a pilot subcarrier at the pth cell forthe cell-specific purposes, the cell-specific information carried bya_(i,p)(t_(k)) and φ_(i,p) (t_(k)) will be of interest to the receiverat the pth cell and other signals described by the second term on theright hand side of equation (1) will be interference, which is anincoherent sum of signals from other cells. In this case, a sufficientlevel of the carrier-to-interference ratio (CIR) is required to obtainthe estimates of a_(i,p)(t_(k)) and Φ_(i,p) (t_(k)) with desirableaccuracy.

There are many ways to boost the CIR. For examples, the amplitude of apilot subcarrier can be set larger than that of a data subcarrier; powercontrol can be applied to the pilot subcarriers; and cells adjacent tothe pth cell may avoid using the ith subcarrier as pilot subcarrier. Allthese can be achieved with coordination between the cells based oncertain processes, described below.

Common Pilot Subcarriers

The common pilot subcarriers for different cells are normally aligned inthe frequency index at the time of transmission, as depicted in FIG. 5,which shows pilot subcarriers divided into two groups: cell-specificpilot sub-carriers and common pilot subcarriers. The cell-specific pilotsubcarriers for different cells are not necessarily aligned infrequency. They can be used by the receiver to extract cell-specificinformation. The common pilot subcarriers for different cells may bealigned in frequency, and possess a set of attributes common to all basestations within the network. Thus, every receiver within the system isable to exploit these common pilot subcarriers without interferenceproblem. The power of the pilot subcarriers can be varied through aparticular power control scheme and based on a specific application.

If the ith subcarrier is used as a pilot subcarrier at the pth cell forthe common purposes, it is not necessary to consider the second term onthe right hand side of equation (1) to be interference. Instead, thisterm can be turned into a coherent component of the desirable signal bydesigning the common pilot carriers to meet the criteria specified inthe aspects of this invention, provided that base stations at all cellsare synchronized in frequency and time. In such a case the cell in whichthe receiver is located becomes irrelevant and, consequently, thereceived signal can be rewritten as:

$\begin{matrix}{{s_{i}\left( t_{k} \right)} = {\sum\limits_{m = 1}^{M}{{a_{i,m}\left( t_{k} \right)}^{{j\phi}_{i,m}{(t_{k})}}}}} & (2)\end{matrix}$

The common pilot subcarriers can be used for a number offunctionalities, such as frequency offset estimation and timingestimation.

To estimate the frequency, normally signals at different times areutilized. In an example with two common pilot subcarriers of the samefrequency index, the received signal at time t_(k+1), with respect tothe received signal at time t_(k), is given by

$\begin{matrix}{{s_{i}\left( t_{k + 1} \right)} = {^{j\; 2\pi \; f_{i}\Delta \; t}{\sum\limits_{m = 1}^{M}{{a_{i,m}\left( t_{k + 1} \right)}^{{j\phi}_{i,m}{(t_{k + 1})}}}}}} & (3)\end{matrix}$

where Δt=t_(k+1)−t_(k). If Δt is much less than the coherence period ofthe channel and

α_(i,m)(t _(k))=c _(i)α_(i,m)(t _(k+1))  (4)

and

φ_(i,m)(t _(k))=φ_(i,m)(t _(k+1))+β  (5)

then the frequency can be determined by

2πf _(i) Δt=arg{s _(i)(k)s _(i)(k+1)}−β_(i)  (6)

where c_(i)>0 and −π≦β_(i)≦π or are predetermined constants for allvalues of m. And from all the frequency estimates {f_(i)}, a frequencyoffset can be derived based on a certain criterion.

For timing estimation, normally multiple common pilot carriers arerequired. In an example of two common pilot subcarriers, the receivedsignal at f_(n), is given by

$\begin{matrix}{{s_{n}\left( t_{k} \right)} = {^{j\; 2{\pi\Delta}\; f\; {T_{s}{(t_{k})}}}{\sum\limits_{m = 1}^{M}{{a_{n,m}\left( t_{k} \right)}^{j\; {\phi_{n,m}{(t_{k})}}}}}}} & (7)\end{matrix}$

where Δf=f_(n)−f_(i) and T_(s) denotes the sampling period. If Δf ismuch less than the coherence bandwidth of the channel and

α_(i,m)(t _(k))=c(t _(k))α_(n,m)(t _(k))  (8)

and

φ_(i,m)(t _(k))=φ_(n,m)(t _(k))+γ(t _(k))  (9)

then T_(s) can be determined by

2πΔfT _(s)(t _(k))=arg{s _(i)*(t _(k))s _(n)(t _(k))}−γ(t _(k))  (10)

where c(t_(k))>0 and −π≦γ(t_(k))≦π are predetermined constants for allvalues of m.

FIG. 6 is an embodiment of pilot-generation-and-insertion functionalblock 106 shown in FIG. 1, which employs a microprocessor 602 togenerate pilot subcarriers and insert them into a frequency sequencecontained in electronic memory 604. In one embodiment of the inventionillustrated in FIG. 6, a microprocessor 602 embedded in thepilot-generation-and-insertion functional block 106 computes theattributes of the pilot subcarriers such as their frequency indices andcomplex values specified by their requirements, and insert them into thefrequency sequence contained in the electronic memory 604, such as aRAM, ready for the application of IFFT.

Diversity for Common Pilot Subcarriers

Considering equation (2), which is the sum of a number of complexsignals, it is possible for these signals to be destructivelysuperimposed on each other and cause the amplitude of the receiversignal at this particular subcarrier to be so small that the signalitself becomes unreliable. Phase diversity can help this adverse effect.In the example of frequency estimation, a random phase υ_(l,m) can beadded to another pilot subcarrier, say the Ith subcarrier, which resultsin

φ_(l,m)(t _(k))=φ_(i,m)(t _(k))+υ_(l,m)  (11)

and

φ_(l,m)(t _(k+1))=φ_(i,m)(t _(k+1))+υ_(l,m)  (12)

where υ_(l,m) should be set differently for each cell, and provided thatthe following condition is met,

φ_(l,m)(t _(k))=φ_(l,m)(t _(k+1))+β_(l), for all values of m  (13)

With the phase diversity, it is expected that the probability of both|s_(i)(t_(k))| and |s_(i)(t_(k))| diminishing at the same time isrelatively small. The embodiment of phase diversity is depicted in FIG.7, which shows common pilot subcarriers generated by a microprocessor ofFIG. 6 to realize phase diversity. It should be noted that time delaywill achieve the equivalent diversity effect.

Another embodiment is illustrated in FIG. 8, which effectively createsphase diversity by adding a random delay time duration 802, either inbaseband or RF, to the time-domain signals.

Power Control for Pilot Subcarriers

In one embodiment of the invention, power control can be applied to thepilot subcarriers. The power of the pilot subcarriers can be adjustedindividually or as a subgroup to

1. meet the needs of their functionalities;

2. adapt to the operation environments (e.g., propagation channels); and

3. reduce interference between cells or groups of cells.

In another embodiment power control is implemented differently forcell-specific pilot subcarriers and common pilot subcarriers. Forexample, stronger power is applied to common pilot subcarriers than tothe cell-specific subcarriers.

Application to Multiple Antennas

The methods and processes provided by this invention can also beimplemented in applications where multiple antennas are used within anindividual sector, provided that the criteria specified either byequations (4) and (5) for frequency estimation or by equations (8) and(9) for timing estimation are satisfied.

FIG. 9 shows two examples for extension to multiple antennaapplications. In case (a) where there is only one transmission branchthat is connected to an array of antennas 902 through a transformer 904(e.g., a beam-forming matrix), the implementation is exactly the same asin the case of single antenna. In case (b) of multiple transmissionbranches connected to different antennas 906 (e.g., in a transmissiondiversity scheme or a multiple-input multiple-output scheme), thecell-specific pilot subcarriers for transmission branches are usuallydefined by a multiple-antenna scheme whereas the common pilotsubcarriers for each transmission branch are generated to meet therequirements of (4) and (5) for frequency estimation or (8) and (9) fortiming estimation.

Joint-Use of Cell-Specific and Common Pilot Subcarriers

In one embodiment the cell-specific and common pilot subcarriers can beused jointly in the same process based on certain information theoreticcriteria, such as the optimization of the signal-to-noise ratio. Forexample, in the estimation of a system parameter (e.g. frequency), someor all cell-specific subcarriers, if they satisfy a certain criterion,such as to exceed a CIR threshold, may be selected to be used togetherwith the common pilot subcarriers to improve estimation accuracy.Furthermore, the common pilot sub-carriers can be used along with thecell-specific subcarriers to determine the cell-specific information insome scenarios, one of which is the operation at the edge of thenetwork.

Base Transmitters Synchronization

Base stations at all cells are required to be synchronized in frequencyand time. In one embodiment of the invention the collocated base stationtransmitters are locked to a single frequency oscillator, as in the casewhere a cell is divided into sectors and the base stations of thesesectors are physically placed at the same location.

FIG. 10 is an embodiment of synchronization in frequency and timedomains of two collocated base stations sharing a common frequencyoscillator 1002. Mobile stations 1004 covered by these two base stationsdo not experience interference when receiving the common pilotsubcarriers. The base station transmitters that are located at differentareas are locked to a common reference frequency source, such as the GPSsignal. FIG. 11 depicts an embodiment of synchronization in frequencyand time domains with base stations 1102 and 1104 at different locationssharing a common frequency reference signal generated from the GPS 1106signals. Mobile stations 1108 covered by these two base stations 1102and 1104 do not experience interference when receiving the common pilotsubcarriers.

In some applications, the entire wireless network may consist ofmultiple groups of cells (or sectors) and each group may have its ownset of common pilot subcarriers. In such scenarios, only those basestations within their group are required to synchronize to a commonreference. While the common pilot subcarriers within each group aredesigned to meet the criteria defined by equations (4) and (5) or by (8)and (9) for the use by its base stations, a particularcounter-interference process (e.g., randomization in frequency or powercontrol) will be applied to different sets of common pilot subcarriers.This will cause the signals from the cells within the same group to addcoherently while the signals from the cells in other groups are treatedas randomized interference.

One embodiment of such implementation is illustrated in FIG. 12, where awireless network consists of three groups (A, B, and C) of cells (orsectors). The base stations within their own group share the same set ofcommon pilot subcarriers. In this scenario, only those base stationswithin their group are required to synchronize to a common reference.While the common pilot subcarriers within each group are designed tomeet the criteria defined in this invention, a particularcounter-interference process (e.g., randomization in frequency) will beapplied to different sets of common pilot subcarriers. For example, thebase stations at Cells Al, A2, and A3 in Group A synchronize to theirown common reference source and transmit the same set of common pilotsubcarriers; and the base stations at Cells B1, B2, and B3 in Group Bsynchronize to their own reference source and transmit another set ofcommon pilot subcarriers that are located at different places in thefrequency domain.

Extension to Transmission of Data Information

All design processes, criteria, and methods described in the embodimentsof this invention can be extended to applications where common networkinformation is required to be distributed to all receivers within thenetwork. In one example, all the base stations within the networktransmit, along with some common pilot subcarriers, an identical set ofdata subcarriers in which the data information common to all the cellsin the network is imbedded.

FIG. 13 shows all base stations within a network transmit, along with acommon pilot subcarrier, a data subcarrier carrying data informationcommon to all cells in the network. A receiver within the network candetermine the composite channel coefficient based on the common pilotsubcarrier and apply it to the data subcarrier to compensate for thechannel effect, thereby recovering the data information.

The above detailed descriptions of embodiments of the invention are notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whilesteps are presented in a given order, alternative embodiments mayperform routines having steps in a different order. The teachings of theinvention provided herein can be applied to other systems, notnecessarily the system described herein. These and other changes can bemade to the invention in light of the detailed description.

The elements and acts of the various embodiments described above can becombined to provide further embodiments.

These and other All of the above U.S. patents and applications and otherreferences are incorporated herein by reference. Aspects of theinvention can be modified, if necessary, to employ the systems,functions and concepts of the various references described above toprovide yet further embodiments of the invention.

Changes can be made to the invention in light of the above detaileddescription. In general, the terms used in the following claims shouldnot be construed to limit the invention to the specific embodimentsdisclosed in the specification, unless the above detailed descriptionexplicitly defines such terms. Accordingly, the actual scope of theinvention encompasses the disclosed embodiments and all equivalent waysof practicing or implementing the invention under the claims.

While certain aspects of the invention are presented below in certainclaim forms, the inventors contemplate the various aspects of theinvention in any number of claim forms. For example, while only oneaspect of the invention is recited as embodied in a computer-readablemedium, other aspects may likewise be embodied in a computer-readablemedium. Accordingly, the inventors reserve the right to add additionalclaims after filing the application to pursue such additional claimforms for other aspects of the invention.

1-12. (canceled)
 13. In a multi-cell, Orthogonal Frequency DivisionMultiple Access (OFDMA) wireless communication system comprising aplurality of base stations in a plurality of cells, a signaltransmission method comprising: arranging multiple adjacent cells in thesystem into a group with an associated group identifier; synchronizingbase stations in the group of adjacent cells in time and frequency to acommon reference source; forming a common set of pilot subcarriers byeach base station in the group of adjacent cells; selectingfrequency-domain signal randomization to be applied on the common set ofpilot subcarriers, wherein the frequency-domain signal randomization isspecific to the group of adjacent cells and different fromfrequency-domain signal randomization applied on pilot subcarriers by abase station outside of the group of adjacent cells; applying thegroup-specific frequency-domain signal randomization on the common setof pilot subcarriers in each base station in the group of adjacentcells; and transmitting the common set of pilot subcarriers by the basestations in the group of adjacent cells using the same frequencysubcarrier indices and the same time indices.
 14. The method of claim13, wherein the base stations are synchronized using Global PositioningSatellite (GPS) signals.
 15. The method of claim 13, where the basestations are synchronized using a common frequency oscillator.
 16. Themethod of claim 13, wherein the common set of pilot subcarriers enable amobile device in the group of adjacent cells to determine compositechannel coefficients for a composite channel, the composite channelcorresponding to an aggregate of different channels from the basestations in the group of adjacent cells to the mobile device.
 17. Themethod of claim 13, wherein the common set of pilot subcarriers enable amobile device in the group of adjacent cells to perform framesynchronization, frequency offset estimation, or timing estimation. 18.The method of claim 13, further comprising transmitting a common set ofdata subcarriers along with the common set of pilot subcarriers by thebase stations in the group of adjacent cells.
 19. The method of claim18, wherein the common set of data subcarriers are transmitted by thebase stations using the same frequency subcarrier indices and the sametime indices.
 20. The method of claim 18, wherein the common set of datasubcarriers carry data information common to the group of cells.
 21. Amulti-cell, Orthogonal Frequency Division Multiple Access (OFDMA)wireless communication system comprising a plurality of base stations ina plurality of cells, the system comprising: an apparatus configured toidentify a group of adjacent cells in the system, the group of adjacentcells having an associated group identifier; an apparatus configured tosynchronize base stations in the group of adjacent cells in time andfrequency to a common reference source; an apparatus configured to forma common set of pilot subcarriers in each base station in the group ofadjacent cells; an apparatus configured to perform frequency-domainsignal randomization on the common set of pilot subcarriers in each basestation in the group of adjacent cells, wherein the frequency-domainsignal randomization is specific to the group of cells and differentfrom frequency-domain signal randomization applied on pilot subcarriersby a base station outside of the group of adjacent cells; and anapparatus configured to transmit the common set of pilot subcarriers inthe base stations in the group of adjacent cells using the samefrequency subcarrier indices and the same time indices.
 22. The systemof claim 21, wherein the base stations are synchronized using GlobalPositioning Satellite (GPS) signals.
 23. The system of claim 21, wherethe base stations are synchronized using a common frequency oscillator.24. The system of claim 21, wherein the common set of pilot subcarriersenable a mobile device in the group of adjacent cells to determinecomposite channel coefficients for a composite channel, the compositechannel corresponding to an aggregate of different channels from thebase stations in the group of adjacent cells to the mobile device. 25.The system of claim 21, wherein the common set of pilot subcarriersenable a mobile device in the group of adjacent cells to perform framesynchronization, frequency offset estimation, or timing estimation. 26.The system of claim 21, further comprising an apparatus configured totransmit a common set of data subcarriers along with the common set ofpilot subcarriers in the base stations in the group of adjacent cells.27. The system of claim 26, wherein the common set of data subcarriersare transmitted by the base stations in the group of cells using thesame frequency subcarrier indices and the same time indices.
 28. Thesystem of claim 26, wherein the common set of data subcarriers carrydata information common to the group of cells.